This invention relates to a pulverized fuel-fired steam generator equipped with a dust collection device and more particularly to a steam air heater for maintaining the flue gas temperature entering said dust collection device above a preselected minimum.
Dust collection equipment is being installed on all pulverized fuel-fired steam generators to remove fly ash and other particulates from the flue gas formed during the combustion process. One common form of dust collection equipment utilized on pulverized fuel-fired steam generators is the cold end electrostatic percipitator, i.e. an electrostatic precipitator located in the flue gas stream immediately downstream of the air heater. One limitation of the cold end electrostatic precipitator is that it cannot be brought into operation until the flue gas temperature entering is above the dew point of water vapor in the flue gas stream as fly ash in the flue gas will have a relatively low resistivity if the moisture in the flue gas condenses. For most pulverized fuel-fired applications, this minimum flue gas temperature required at the electrostatic precipitator is 200.degree. F. Another problem associated with cold end electrostatic precipitators is a loss in efficiency due to the presence of condensed sulfuric acid mist in a flue gas. Even a few ppm's of condensed sulfuric acid in the flue gas passing through the electrostatic precipitator can result in halving of its collection efficiency. Further, optimum electrostatic precipitator performance is obtained with a flue gas temperature of 275.degree. to 325.degree. F., the range of maximum fly ash resistivity. For these reasons, operation of the electrostatic precipitator is precluded during start-up and low load operation and a more expensive fossil fuel such as oil or natural gas must be fired during start-up in order to meet stack opacity requirements and particulate emission requirements, with pulverized fuel not being fired until the required minimum flue gas temperature is reached.
Another highly efficient form of dust collection equipment is the fabric filter. However, the application of fabric filters to pulverized fuel-fired steam generators has been somewhat limited because of considerations relating to flue gas temperature. If the flue gas temperature entering the fabric filter drops below the dew point of water vapor, the fly ash collected on the bag filter is moist and tends to adhere to the filter bags causing an adverse increase in pressure drop. Also, if the flue gas temperature entering drops below the condensation point of sulfuric acid, the flue gas becomes corrosive to all but prohibitively expensive bag fabrics and bag life is greatly reduced. For these reasons, use of fabric filters on pulverized fuel-fired steam generators is precluded during start-up and low load operation unless a more expensive fossil fuel such as natural gas is fired.
Recent technological innovations with respect to igniting pulverized coal now make it possible to light-off and stabilize a steam generator on coal exclusively. Coal-fired warm-up nozzles replace the standard oil- or gas-fired warm-up guns and ignition energy is supplied by a high energy electric arc from a spark plug type ignitor integral to the coal-fired warm-up nozzle. Thus, it is theoretically possible to eliminate the use of the more expensive and less available fossil fuels such as oil and natural gas during start-up and low load operation on coal-fired boilers. However, a practical consideration limiting the utilization this technology is the unavailability of dust collection equipment during start-up and low load operation for the reasons discussed previously. Due to the ash content of coal, particulate formation during coal-fired start-up and low load operation would preclude the use of coal because of environmentally unacceptable levels of particulate emission and stack opacity unless dust collection equipment is on-line.
In the invention disclosed herein, a steam air heater is disposed in the combustion air intake duct between the forced draft fan and the main flue gas heated air preheater for raising the temperature of the combustion air entering the main air preheater. It is known in the prior art to utilize a steam air heater so disposed to raise the temperature of the combustion air entering the main air preheater on pulverized fuel steam generators during low load operation in order to limit the corrosion of the cold end heat transfer surfaces on the main air preheater. However, the incoming combustion air is heated only to such a temperature as necessary to insure that the average metal temperature in the cold end section of the main air preheater remains above a specified minimum temperature, said minimum temperature being a function of the flue gas composition and the material used in the construction of the main air preheater. The amount of steam passed through the steam air heater is regulated so as to maintain the average cold end temperature above the specified minimum metal temperature. The average cold end temperature is defined as one half the sum of the air temperature entering and the flue gas temperature leaving the main air preheater and ranges from 155.degree. to 185.degree. F. for coal firing. In accordance with the invention disclosed herein, the flow rate of the steam passed through the steam air heater is regulated in response to the flue gas temperature entering the dust collection equipment and not, as in the prior art, in response to the average cold end temperature.